The fate of pebbles and planetesimals entering protoplanetary envelopes

Sammanfattning: Planetary embryos grow by the accretion of solid dust-material, ranging from cm- to m-sized pebbles up to km-sized planetesimals. However, the underlying size-distribution of the accreted material is poorly understood. When the pebbles and planetesimals encounter a protoplanet, they are subjected to the gaseous environment of the protoplanetary envelope. Because of the drag-force from the gas, the pebble- and the planetesimal-trajectories change significantly from their initial Keplerian orbits, and so does the evolution of their surface temperatures and the ablation rates. It is consequently of interest to track the evolution for a large range of particle sizes that encounter protoplanets, from small pebbles to large planetesimals. Depending on which particle size is responsible for the growth of protoplanets, and the corresponding thermal evolution and ablation, the protoplanetary core and its envelope will evolve differently. If all the accreted particles are ablated, we expect the envelopes of protoplanets to be polluted and with less massive cores. On the other hand, if the ablation is inefficient, protoplanetary cores are expected to grow efficiently. Effectively, the form in which material is accreted sets constraints on protoplanetary interior and atmospheric evolution models. In this project, I study the evolution of both pebbles and planetesimals that encounter a protoplanet. This is done by simulating the trajectories of the in-falling solids in the protoplanetary envelope while tracing their thermal evolution, dynamical pressure, and ablation. The mass loss of the particles is further related to the accretion rates onto protoplanets of different mass, where both the solid and the ablated mass are accounted for. From the results, I can conclude that pebbles are efficiently ablated above protoplanetary cores with masses of 0.5 M_\oplus. Small planetesimals, between 10^3-10^4 cm in size, are fully ablated for core masses about 1-5 M_\oplus. For sizes of 10^5-10^6 cm, the core masses have to reach between 5-10 M_\oplus. Finally, for planetesimals on the order of 10^7-10^8 cm, several tenths of Earth-masses are required to fully ablate the impactors. This means that if protoplanets grow predominantly by pebble accretion, they grow into, so called, vapour-blobs, already at 0.5 M_\oplus. The evolution of the protoplanetary interior structure and the pollution of the envelope and its chemical composition is consequently determined by the internal gas-flows, dust settling, and the interchange of material between the envelope and the protoplanetary disc. I also find that the latent heat is cooling the surface of the impactors efficiently, limiting the ablation rates. Planetesimals, that have a large reservoir of volatiles, can remain cold as they pass through the envelope by only ablating a small fraction of their total mass. I further find that the sum of the accreted solid mass and the ablated material follows the classical core accretion model, where no ablation of the particles is included. Thus, the results obtained in classical core accretion simulations are in the larger picture unaffected by ablation.